Coxiella burnetii is a Gram-negative bacterium and causative agent of the human disease Q fever. This intracellular pathogen has evolved complex mechanisms for evading host cell defenses. One important feature that makes C. burnetii unique is that it survives within the acidic and hydrolytic environment of host lysosomes and modifies this organelle to create a large vacuole that supports replication. To accomplish this, C. burnetii encodes a Dot/Icm type IV secretion system that delivers over 100 effector proteins into the cytosol of the host cell during infection. These effectors modulate host cell functions, which include manipulating host vesicular transport pathways to promote membrane fusion and expansion of the vacuole containing C. burnetii. Host determinants required for biogenesis and fusion of the C. burnetii-containing vacuole (CCV) have not been clearly defined; however, because Rab GTPases coordinate vesicular transport within host cells, and are often modulated by bacterial effector proteins, we hypothesize that Rab proteins are central to the infection strategy used by C. burnetii. This hypothesis is supported by data showing that the silencing of specific Rab proteins in host cells will interfere or enhance C. burnetii replication. Here, we propose studies to determine the spatiotemporal roles for Rab GTPases during C. burnetii infection, and to determine the Rab-directed processes that are required to transform a lysosome-derived vacuole into a unique intracellular compartment that supports C. burnetii replication. We will disrupt the function of specific Rab proteins and determine the stage of CCV maturation that is regulated by the Rab. We will also define the cohort of Rab proteins that localize to the CCV during these specific maturation stages. Together, this will reveal the Rab proteins that are required for C. burnetii transport to a lysosome-derived vacuole, acidification o the vacuole, activation of the Dot/Icm secretion system, and the import of host factors required for bacterial replication. We will also focus on Rab proteins that negatively impact C. burnetii intracellular replication by determining how these Rab proteins affect CCV biogenesis and intracellular survival of C. burnetii and testing whether C. burnetii effector proteins required fo intracellular infection have activities that affect the function of these Rab proteins. These studis will elucidate host membrane transport pathways that are used to create the intracellular environment that supports C. burnetii replication and will identify Rab proteins that have important roles in defending host cells against intracellular pathogens.